1. Field of the Invention
The present invention relates to an acoustic wave filter including a plurality of acoustic wave elements connected to each other and an acoustic wave filter device. More specifically, the present invention relates to a midpoint-grounded balanced acoustic wave filter having a balance-unbalance converting function and an acoustic wave filter device.
2. Description of the Related Art
Conventionally, various surface acoustic wave (SAW) filters have been used as band-pass filters for communication apparatuses, such as mobile phones. In particular, a SAW filter used in an RF (radio frequency) stage of a mobile phone is required to have a balance-unbalance converting function. When having the balance-unbalance converting function, a SAW filter does not need to include a balun functioning as a balance-unbalance converting element. Accordingly, the mobile phone or other suitable device can be miniaturized.
Japanese Unexamined Patent Application Publication No. 2004-88551 (Patent Document 1) discloses an example of a midpoint-grounded balanced SAW filter having a balance-unbalance converting function. FIG. 24 is a schematic plan view showing the SAW filter described in Patent Document 1, and FIG. 25 shows the circuit configuration thereof.
As shown in FIG. 24, a SAW filter 200 includes a surface wave substrate 301. First and second SAW filter elements 201 and 202 and first and second SAW resonators 203 and 204 are disposed on the surface wave substrate 301. As shown in FIG. 25, the first filter element 201 is a longitudinally-coupled SAW filter element including IDTs (interdigital transducers) 205 to 207 and reflectors 208 and 209 aligned in a propagation direction of surface waves. The filter element 202 has the same configuration, including IDTs 210 to 212 and reflectors 213 and 214. Note that the phase of the second filter element 202 is 180 degrees different from that of the first filter element 201.
The center IDT 206 of the first filter element 201 and the center IDT 211 of the second filter element 202 connect to an unbalanced terminal 215. One end of the IDTs 205 and 207 connect to a first balanced terminal 216 via the SAW resonator 203. On the other hand, one end of the outer IDTs 210 and 212 of the second filter element 202 connect to a second balanced terminal 217 via the SAW resonator 204. With this configuration, the SAW filter 200 functions as a balanced SAW filter including the unbalanced terminal 215 and the first and second balanced terminals 216 and 217.
In the first and second filter elements, the ends opposite to the ends connected to the unbalanced terminal 215 of the IDTs 206 and 211 are grounded. As shown in FIG. 24, the IDTs 206 and 211 connect to electrode pads 302 and 303 provided on the surface wave substrate 301, respectively. The electrode pads 302 and 303 are electrode pads connected to a ground potential.
During manufacturing, metallic bumps are provided on the electrode pads 302 and 303, and the metallic bumps are electrically connected to an electrode land connecting to a ground potential on a package side. Accordingly, the electrode pads 302 and 303, that is, the grounded ends of the IDTs 206 and 211, are mutually connected on the electrode land connecting to the ground potential on the package side.
On the other hand, the ends opposite to the ends connected to the balanced signal terminal 216 of the IDTs 205 and 207 of the first filter element are also grounded. That is, in FIG. 24, the grounded end of the IDT 205 is electrically connected to an electrode pad 311. In addition, the IDT 207 is electrically connected to an electrode pad 312. The electrode pads 311 and 312 are electrode pads connected to the ground potential.
Likewise, on the side of the second filter element 202, the ends opposite to the ends connected to the balanced signal terminal 217 of the IDTs 210 and 212 are grounded. More specifically, as shown in FIG. 24, the grounded end of the IDT 210 is connected to the electrode pad 312, whereas the grounded end of the IDT 212 is connected to an electrode pad 313. Similar to the above-described electrode pads 302 and 303, the electrode pads 311 to 313 are connected to an electrode land that is provided on the package substrate and that is connected to the ground potential. That is, the electrode pads 311 to 313 are mutually connected as shown in the package substrate.
The SAW filter 200 is mounted on the package substrate by a face-down method. In this case, metallic bumps are provided on a plurality of electrode pads including the above-described electrode pads 302, 303, and 311 to 313. Then, the SAW filter 200 is mounted on the package substrate such that the surface on which the metallic bumps are provided is a lower surface, so that the metallic bumps enable the electrode pads 302, 303, and 311 to 313 to be connected to the electrode lands on the package substrate.
Since the above-described SAW filter 200 is assembled using a face-down method and does not need to include a balun because of having a balance-unbalance converting function, the SAW filter 200 can be miniaturized, so that the RF stage including the SAW filter 200 can also be miniaturized.
However, when the SAW filter 200 is actually used as a balanced SAW filter having a balance-unbalance converting function while being mounted on the package substrate as described above, out-of-band attenuation is not sufficient.